1. Field of the Invention
The present invention relates to a light guide plate and a backlight unit, which are used in a transmissive or semi-transmissive liquid crystal display apparatus, an advertisement plate, an emergency guide light and the like.
2. Description of the Related Art
In recent years, color liquid crystal display (LCD) apparatuses are widely used in various devices such as a portable telephone, a portable personal computer, a portable liquid crystal television or a video integrated liquid crystal television and the like. The LCD apparatus is basically provided with a backlight unit and a liquid crystal element. Configuration of the backlight unit can be roughly classified into a direct type and an edge light type. For the direct type backlight unit, a light source is placed just beneath a liquid crystal element. For the edge light type backlight unit, a light source is placed on a side of a translucent light guide plate so that a light is emitted from the entire surface of the light guide plate. From the viewpoint of downsizing the LCD apparatus, the edge light type backlight unit is commonly used.
In such LCD apparatus, it is required to extend the operating time of the battery. However, the backlight unit used in the LCD apparatus consumes a high proportion of the electric power, which is an obstacle for extending the operating life of the battery. In order to extend the operating life of the battery and to improve the practical value of the LCD apparatus, it is very important to reduce the electric power consumption of the backlight unit as much as possible. However, if luminance of the backlight unit is decreased by reducing the electric power consumption of the backlight unit, display quality of the LCD apparatus may undesirably deteriorate. Therefore, in order to reduce the electric power consumption without decreasing the luminance of the backlight unit, development of a backlight unit having high luminance efficiency and evenness is advancing.
Currently, the most popular backlight unit includes a light source 1, a light guide plate 2, a diffusion film 3, upward prism sheets 4, 5 and a reflection sheet 6, as shown in FIG. 1. The light emitted from the light source 1, such as a light emitting diode (LED) and the like, enters in the light guide plate 2 from an entrance surface of the light, which is an end surface 2a of the light guide plate 2, and is guided inside the light guide plate 2. The light is reflected by a reflection element 122 having a plurality of grooves, a plurality of dots and the like. The reflection element 122 is formed on a reflection surface. The reflection surface is a bottom surface 2b of the light guide plate 2. The light is emitted in an oblique direction from an emitting surface defined by a top surface 2c of the light guide plate 2. The arrangement of the reflection element is designed such that the in-plane distribution of the luminance is even. For example, a surface density of the reflection element is low in the vicinity of the light source 1, and the surface density becomes higher as the distance from the light source 1 becomes longer. Consequently, the LCD apparatus has an even luminance.
However, the light is emitted in the oblique direction from the light guide plate 2. Thus, in order to effectively use the light, it is necessary to deflect and collect the light in a normal direction of the backlight unit. Therefore, the diffusion sheet 3 is placed on the light guide plate 2 so as to improve the evenness and to deflect the output light from the light guide plate 2 in the normal direction of the backlight unit. Moreover, as the lens sheet for controlling the direction of the lights and collecting the lights, two laminated upward prism sheets 4, 5 are provided. The prism sheets have a plurality of prism columns, each of which has a triangular cross section with an apex angle of about 90°, and are disposed on the diffusion sheet 3. The upward prism sheets 4, 5 are laminated such that each array direction of the prism columns is orthogonal to each other, so as to improve luminance efficiency of the backlight unit.
In the orthogonal configuration in which each array direction of the prism columns of the upward prism sheets 4, 5 is orthogonal to each other, directional control is performed for deflecting the emitted light from the light guide plate 2 to the normal direction of the backlight unit mainly by refraction on slant surfaces of the prism columns. Therefore, since apart of the light is laterally reflected and refracted, it is difficult to improve luminance efficiency. On the other hand, another part of the light is totally reflected in a downward direction out of the light guide plate 2. This light may be reflected by the reflection sheet 6 placed on a backside of the backlight unit and can be reused. The reused light may be emitted from a different position from the reflected position of the light guide plate 2. Thus, it is effective for resolving in-plane uneven luminance and increasing uniformity of the luminance. Since the arrangement shown in FIG. 1 provides a good balance between the efficiency and evenness of the luminance, this arrangement is widely used.
However, since the backlight unit shown in FIG. 1 is provided with one diffusion film and two prism films, the number of parts is large. Therefore, the backlight unit shown in FIG. 1 has disadvantages in that fabrication cost of the backlight unit increases due to cumbersome assembling work, and thickness of the backlight unit increases.
A method for decreasing a number of parts of the backlight unit has been proposed (refer to JP-A H07-198913 (KOKAI)). In the proposed backlight unit, a double-sided prism sheet 11 shown in FIG. 2, which is used instead of the prism sheets 4, 5 in the backlight unit shown in FIG. 1, is provided on the diffusion sheet 3. On respective top and bottom surfaces of the prism sheet 11, prism columns whose array directions differ from each other are provided. The proposed backlight unit has an advantage in decreasing the number of parts of the backlight unit by one, since the prism sheet, in which functions of the two prism sheets are integrated into one, is used in combination with the diffusion film.
When using the lens sheet including stacked two upward prism sheets in which each prism column is arranged orthogonal to each other, Fresnel reflection loss occurs at the interface between the prism sheet on the output side and the air gap between the prism sheets. In the proposed backlight unit, the air gap between the prism sheets is eliminated by using the double-sided prism sheet 11 having the prism columns on the top and bottom surfaces. Thus, it is possible to prevent the Fresnel reflection loss and to increase luminance of the backlight unit.
However, in the downward prism columns provided on the bottom surface of the double-sided prism sheet, a slant surface of each prism column mainly acts as a total internal reflection plane. Since the output light from the diffusion film has a low directivity, the downward prism columns have a disadvantage in that efficiently directional control of the light in a predetermined direction is not possible. As a result, on each slant surface of the upward prism columns provided on the top surface of the double-sided prism sheet, reflection and refraction components of the light to the lateral direction may increase. Thus, there is a problem such that luminance of the backlight unit may decrease compared with the backlight unit including the lens sheet having two upward prism sheets in which the array directions of respective prism columns of the two upward S prism sheets are orthogonal to each other.
A backlight unit having a downward prism sheet used as a lens sheet has been proposed (refer to JP No. 2739730). The proposed backlight unit is designed such that the diffusion film 3 and two upward prisms 4, 5 shown in FIG. 1 are replaced with a downward prism sheet 21, as shown in FIG. 3. The downward prism sheet 21 has a plurality of prism columns, each of which has a triangular cross section. The prism sheet 21 is downwardly disposed so that the prism columns face the top surface 2c of the light guide plate 2. The array direction of the prism columns is parallel to the end surface 2a of the light guide plate 2. The obliquely directed light emitted from the light guide plate 2 is refracted on a slant surface of each prism columns and totally reflected at another slant surface in the normal direction of the backlight unit. Thus, the emitted light from the light guide plate 2 is deflected in the normal direction of the backlight unit. In the configuration using the downward prism sheet 21, the directional light emitted from the light guide plate 2 is directly and totally deflected in the normal direction of the backlight unit. Thus, the front luminance efficiency may be increased in principle.
Furthermore, the number of parts in the lens sheet can be reduced to only one downward prism sheet 21. However, since the emitted light has a high directivity, it is difficult to reduce unevenness of the entering light and to assure even luminance. In practice, the diffusion film is stacked on the downward prism sheet 21 in many cases.
Recently, in the display of portable devices, LEDs are usually used as the light source. As shown in FIG. 4, in case of using LEDs as the light source 1, when the backlight is turned on and viewed from the front, an uneven light entrance region 33, in which a dark portion 31 and a bright portion 32 are clearly split due to the directional characteristics of the light emitted from the LEDs, occurs in the vicinity of the light entrance portion of the backlight. When reducing thickness and downsizing of the devices, the area ratio of a display area 34 in the backlight unit tends to be increased, and a distance LL of a light entrance area 35 between the entrance surface 2a and the end of the display area 34 is decreased. For this reason, minimization of the unevenness of the entering light is also an important subject.
The backlight unit of the downward prism sheet configuration disclosed in JP No. 2739730 has a disadvantage in that, since the output light from the light guide plate is emitted in the normal direction directly with one deflection without any redirect by reflection, a region where the uneven light is visible in the vicinity of the light entrance portion may increase. Also, even if the diffusion sheet is stacked on the downward prism sheet to improve the light unevenness, the light unevenness cannot be effectively reduced. Thus, in the actual situation, the configuration using a downward prism sheet is limited to the backlight unit in which the non-display area is large.